1. Field of the Invention
This invention relates to a small-size disc cartridge comprising a magnetic disc housed for rotation in a flat casing having a metal shell, and more particularly to a method of carrying out plastic processing involving stretching such as barring on the metal shell or other metal plate members and a disc cartridge whose tubular shaft is formed by the method.
2. Description of the Related Art
Recently, magnetic discs are increasingly given a large capacity and are increasingly made in a small size. Structural parts of a magnetic disc cartridge housing the magnetic disc for rotation have been complicated in shape and at the same time, a high dimensional accuracy of the structural part has been required.
Thin metal plate members are often used as structural parts of a magnetic disc cartridge by processing based on their stretchability. Barring and pressing which are common methods of processing a metal plate are also based on stretchability of a metal plate and are used, for instance, for forming a projection, forming a bent portion or forming a curved surface.
FIGS. 8A to 8E are for illustrating an example of barring employed when a projection is formed in a metal plate. A small diameter hole 21 is first formed in a metal plate member 20 as shown in FIG. 8A. Then a smaller diameter portion 31 of a barring tool 30 comprising cylindrical smaller and larger diameter portions 31 and 33 provided on opposite end portions of a conical body portion 32 which is a truncated cone in shape is inserted into the hole 21 as shown in FIG. 8B.
When the tool 30 is subsequently forced, a part 22 of the metal plate member 20 circumscribing the hole 21 is expanded upward as shown in FIG. 8C as the conical body portion 32 enters the hole 21, and further expanded and stretched to be plastically deformed as the larger diameter portion 33 enters the hole 21 as shown in FIG. 8D. The metal plate member 20 is finally processed to a member having a cylindrical portion 23 projecting upward by a height h, the distance between the upper surface 20a of the metal plate member 20 and the upper surface 23a of the cylindrical portion 23 as shown FIG. 8E and FIG. 9A. Such a cylindrical portion is formed in the metal shell of a disc cartridge for supporting for rotation the rotary shutter as will be described later.
However, when the limit of stretching of the metal material is exceeded during barring of such a cylindrical portion 23, cracks C are produced as shown in FIG. 9B, which can deteriorates quality and/or yield of the product. The cracks C are apt to be produced in the cylindrical portion 23 near the upper surface 23a thereof as shown in FIG. 9B. Especially when the cylindrical portion 23 functions as a tubular shaft for supporting another part for rotation, the cracks C are expanded and/or increased in number, for instance, when the upper end portion of the cylindrical portion is caulked to form a flange for preventing dismounting of said another part from the cylindrical portion 23.
Further, when a curved surface portion 25 is formed in the metal plate member 20 by drawing as shown in FIG. 10A, cracks C are apt to be produced in a central portion of the curved surface portion 25 as shown in FIG. 10B.
Such cracks C can produce metal fractions, which can seriously damage the disc cartridge. Production of such cracks C depends upon the thickness and/or the stretchability of the metal material, and/or the processing speed and it is very difficult to control the production site and/or the size of the cracks. Conventionally, production of such cracks C has been suppressed by enhancement of the limit of acceptance of the material and/or suppression of fluctuation in the processing speed. However such provision is sometimes not effective and at the same time, adds to the cost.